In ink-jet printing methods, droplets of ink are directly projected from very fine nozzles and allowed to adhere to a recording medium to form printed materials having characters or images printed on the recording medium, etc. The ink-jet printing methods have become rapidly spread because of various advantages such as easiness of full coloration, low cost, capability of using a plain paper as the recording medium, non-contact with printed characters or images on the recoding medium, etc. In particular, form the viewpoint of good weathering resistance and water resistance of the resulting printed materials, the ink-jet printing methods using an ink containing a pigment as a colorant has become predominant.
The pigment used in the ink is dispersed in an ink vehicle using a polymer dispersant. However, since molecules of the pigment are incapable of being uniformly dissolved in the ink vehicle unlike dyes, it is necessary to improve storage stability of the ink for maintaining a good dispersed state of the pigment in the ink, and suppress deterioration in ejection properties of the ink owing to solidification of the polymer or pigment in portions of ink ejection nozzles which tends to be caused by a poor dispersed state of the pigment in the ink after a long-term storage or use of the ink.
For example, WO 1999/052966A (Patent Literature 1) aims at obtaining a pigment dispersion that is excellent in water resistance, stability during the passage of time, etc., and discloses a water-based pigment dispersion obtained by dispersing a pigment with a thermoplastic resin containing a carboxy group at a specific ratio and then crosslinking the carboxy group-containing thermoplastic resin with a crosslinking agent, in which the ratio of the crosslinking agent to the carboxy group-containing thermoplastic resin (crosslinking agent/carboxy group-containing thermoplastic resin; in terms of a weight ratio of effective solid matters) in the dispersion is from 1/100 to 50/100. In Patent Literature 1, it is further described that the thermoplastic resin is preferably neutralized with an organic amine having a boiling point of at most 200° C., and the neutralization rate of the thermoplastic resin is preferably from about 100 to about 150%.
On the other hand, in order to solve the aforementioned problems by enhancing dispersion stability of the pigment in the ink vehicle, it is known to use a so-called self-dispersible pigment on a surface of which a chemical structure capable of allowing the pigment to exhibit good dispersion stability is directly formed without using any polymer dispersant. In the case of using such a self-dispersible pigment in the ink, although it is possible to suppress solidification of the pigment in the portions of ink ejection nozzles, there tends to arise such a problem that the resulting ink suffers from damage to water resistance thereof as an advantage attained by using the pigment in the ink.